1. Field of the Invention
This invention relates generally to network synchronization and clock recovery, and in particular, to packet networks that transmit and receive isochronous data. More particularly, the present relates to a network node that maintains network synchronization utilizing Synchronous Residual Time Stamp (SRTS), where reference timing is derived from local clocks available at the source and destination nodes as opposed to a common end to end network clock.
2. Description of Related Art
Packet networks are convenient for transferring time insensitive data, such as computer files, between remote nodes. When it is desired to send time-sensitive or isochronous data, such as voice and video, over a packet network, some means must be found to transport the service clock, namely the clock that originates with the isochronous service, over the network. The characteristics of this clock as well as the accompanying isochronous data should be transported over the network without actually sending the clock signal itself.
The packet network uses a common reference clock, known as the network clock, to clock the data over the packet network. Sometimes, but not always, the network clock is made available to the source and destination nodes.
Each node generates a local clock with a digitally controllable frequency. This is used to regenerate the service clock at the receiving node.
The transfer of isochronous voice and video data over a packet network between nodes requires that the node clocks be synchronized so as to prevent data loss due to slips. A slip can be defined as an overflow or underflow of data buffers, which are typically designed to absorb jitter and wander (low frequency clock variation). Slips in video signals degrade visual performance, and it is thus important to reconstruct the source synchronization with high accuracy. Clock slips in digital voice connections cause clicks and pops that degrade audio performance. The accumulation of jitter and wander in voice networks must be controlled in order to ensure a high quality of service. The required accuracy of a recovered clock at the slave end of a packet network may depend upon the requirements of the rest of the network that this clock has to synchronize.
Several methods exist for the transport of clock information over packet networks, as a means to provide synchronized clocking at either end of the network for isochronous services (e.g. voice and video). The most notable methods are the Plesiochronous mode, Synchronous Residual Time Stamp (SRTS) (or variant RTS method), or the Adaptive Clock Receiver (ACR) method. The SRTS method is generally preferred when a common end to end network clock is available, and the ACR method is often alternatively chosen when a common network clock is not available. The Plesiochronous mode may be used when a traceable stratem-1 clock source is available at both ends of the network, for example when a GPS clock is available. Both SRTS and ACR methods are used extensively in ATM networks, the ACR method being used more and more due to the lack of a synchronized end to end network clock. Both methods may also be used for other types of packet networks e.g. IP networks with Ethernet layer 2, although the synchronous network clock is rarely available with Ethernet. Prior art clock recovery methods appear to use one of the above methods, or may select one of them at any given time as needed.
In the SRTS method, timing information is carried through the network with the data transmission. The destination node uses this timing information to recover the frequency of the source node service clock, which determines the frequency of the destination node service clock. SRTS must have a common clock available at both ends of the packet network. The SRTS method is based on the coding of the frequency difference between the service clock and a network reference clock into a Residual Time Stamp. This Residual Time Stamp is coded inside the packet headers, and transported to the other side of the network. The same frequency difference is reproduced on the other side of the network, reproducing the service clock at the receive node.
ACR provides the recovery of the master side clock frequency at the slave node without the use of a common network clock. The distribution of a common network clock is not usually possible in Ethernet networks for example.
The ACR method is generally based on the fill level of a buffer receiving the incoming data traffic. The local frequency is adjusted so as to keep the fill level of the buffer at a more or less constant level (e.g. half full). Other methods of ACR have also been published where the long term average of inter-packet timestamp arrival times is averaged, compared with locally generated timestamps, and filtered to provide an error correction control to the frequency of the local oscillator.
SRTS has the advantage that it generally provides a higher accuracy of clock recovery than does ACR. SRTS does not rely on statistics of the cell or packet jitter except that it has a known, bounded amplitude. Therefore, the recovered clock has the capability of a high degree of frequency stability unaffected by cell or packet delay variation, and it is capable of transferring the wander characteristics of the service clock (which is important).
A drawback to the SRTS clock recovery method is that it assumes that a common network reference clock is provided to the source and destination nodes. A common network reference clock is often not available for several reasons. Each portion of the network may be a separate timing domain, and would therefore be synchronized to a different reference clock. Multiple interconnected ATM networks are an example, because the separate ATM networks will not use the same clock. IP networks that use Ethernet are another (more extreme) example, where each network hop may use a different physical clock, the difference in timing being compensated by the insertion of inter-frame idle data. A loss of synchronization could occur, and in this case the network will continue to operate using a holdover clock sourced locally within a network node, i.e. not traceable to a PRS.
ACR has the advantage that it does not need a common network clock, but it has the disadvantage that it must attempt to filter out the statistics of packet delay variation. This requires a phase locked loop with a loop filter with very low cut-off frequency. There is a trade-off between filter time constant (which affects convergence time), and wander performance. It is often necessary to transfer the wander characteristics of the service clock rather than attempt to filter it out. ACR can actually add large amounts of wander to the transported service clock, due to changing network packet delay variation statistics.
The following U.S. Pat. Nos. relate generally to such prior art SRTS and ACR systems: 5,260,978; 6,122,337; 5,742,649; 5,896,427; 5,396,492; 6,157,646; 5,812,618; 6,026,074; 6,144,714; 6,167,048; 5,822,383; 6,044,092; 5,912,880; 5,740,173; 5,825,750; 6,046,645; 6,111,878; 6,137,778; 6,144,674; 6,195,353; 6,011,823; 5,608,731; 5,896,388; 6,108,336.
A full description of SRTS can be found in Synchronous Residual Time Stamp (SRTS), as described in ITU-T I.363.1 B-ISDN ATM Adaptation Layer specification: Type 1 AAL. Pages 13-16, the contents of which are herein incorporated by reference.